1. Field of the Invention
The present invention relates to a technique for improving the productivity of a semiconductor device and, more particularly, to a display device having a circuit composed of a thin film transistor (TFT), and a method of manufacturing the display device. The present invention further relates to a liquid crystal display device and an electronic device having the liquid crystal display device mounted thereon.
2. Prior Art
In recent years, there has been noted the technique for constructing the TFT using a semiconductor film (having a thickness of about several to several hundreds nm) formed over a substrate having an insulating surface. The TFT is widely applied to an electronic device such as an integrated circuit (IC) or an electrooptical device and has been urgently developed as a switching element for a liquid crystal display device.
The liquid crystal display device is widely used as a display device having advantages in light weight and small thickness and in low power consumption, such as a mobile terminal, e.g., a note personal computer in the business aspect or the monitor of a personal computer or a thin TV in home. The liquid crystal display device is taking reliable place of the CRT which has been a major role in the display device.
Generally, the display device is the means for making the information visually recognizable such as the image transformed and transmitted in electric signals, so that the electric signals can be adversely transformed into the optical signals and reconstructed into the image. There have been made a number of such display devices including the liquid crystal display device.
The liquid crystal display device is a mechanism which is enabled as an electric shutter or valve to control the transmission or non-transmittance of a light emitted from a light source, by using the electric or optical anisotropy owned by the liquid crystal, so that the electric image signals applied to the display device may be made visible.
In order that the optical anisotropy owned by the liquid crystal may be effectively used to make visible the electric signals applied to the liquid crystal, the liquid crystal molecules are oriented in a predetermined state in the liquid crystal display panel (or the liquid crystal panel). The method of applying the electric signals and the orientation of the liquid crystal molecules are closely correlated to each other, and several methods have been proposed heretofore. Generally, these operating methods are called the “operation modes”. The operation modes thus far proposed are represented by: the twisted nematic (TN) mode using the nematic liquid crystal; the vertical alignment (VA) mode; the in-plane switch (IPS) mode; the surface stabilized-ferroelectric liquid crystal (SSFLC) mode using the smectic liquid crystal such as a ferroelectric liquid crystal or an anti-ferroelectric liquid crystal; or a tri-state switching mode.
The liquid crystal display device using these operation modes is constructed with a view to realizing and keeping the homogeneity of the picture quality, such that the spacing of a pair of substrates can be held uniform so as to improve the in-plane homogeneity of the transmittance characteristics of the liquid crystal display panel (or the liquid crystal panel) to the applied voltage or so as to improve the response characteristics homogeneity of the liquid crystal to the applied voltage by the entirety of the liquid crystal display device. Where the smectic longitudinal direction, as represented by the non-threshold value anti-ferroelectric liquid crystal or a mono-stabilized ferroelectric liquid crystal, is applied to the display device, moreover, it is essential to make a remarkably narrow cell gap (1 to 2 microns).
In order to realize this, the liquid crystal display device combines a polarizing plate and a back light, if necessary, and by incorporating the integrated structure into the display unit so as to exploit the structure.
FIG. 13 presents a top plan view of the liquid crystal display device of the prior art and a sectional view taken along a dotted line A-A′ from the top plan view. This liquid crystal display device is constructed such that a liquid crystal 1014 is held at a constant spacing of about 10 microns or less by a paired substrates 1000 and 2001, one of which is transparent. In order to apply an electric field to the liquid crystal 1014, moreover, a pixel electrode 1007 made of a conductive thin film is formed over the surface of the substrate 1000, and an opposed electrode 1006 made of a conductive thin film is formed over the surface of the substrate 2001. The substrate 1000 having a display pixel portion 1003, a peripheral drive circuit 1004 and an external leading wiring line portion 1005 will be called the “active matrix substrate 1001”. The display pixel portion will also be merely called the “pixel portion”. The peripheral drive circuit 1004 will be a general name of a gate wiring line side drive circuit 1004a and a source wiring line side drive circuit 1004b. In an X-direction from the gate wiring line side drive circuit 1004a, there are formed a plurality of gate wiring lines (although not shown) in the display pixel portion 1003. In a Y-direction from the source wiring line side drive circuit 1004b, there are formed a plurality of source wiring lines (although not shown) in the display pixel portion 1003. A layer insulating film (although not shown) is provided between the gate wiring line and the source wiring line. Over the substrate 1000 and the substrate 2001, respectively, there are formed an orientation film 1010 and an orientation film 1011.
In order to make uniform the substrate spacing between the active matrix substrate 1001 and an opposed substrate 1002, moreover, there are mounted on the substrates a number of spacers 1009 which are given an equal size against the force in a direction to narrow the substrate spacing. Moreover, it is an ordinary method to fix the substrates with a seal member 1012 so that the substrates may not separate each other. The seal member is exemplified by a thermoset resin such as an epoxy resin or a UV resin to be set with an ultraviolet ray. However, the seal member is made of an insulating material.
In order to adhere the active matrix substrate 1001 and the opposed substrate 1002, moreover, the seal member 1012 is formed to extend along the peripheral portion (or the external peripheral portion) of the region in which the paired substrates overlap, and to enclose at least the display pixel portion 1003. The seal member 1012 is made of an adhesive. It is ordinary to set the line width of the pattern of the seal member 1012 to a constant value of about 1 to 4 mm. The seal member 1012 has an additional function to seal the liquid crystal from leaking to the outside of the panel other than that for the adhesion.
In order to arrange the liquid crystal in the liquid crystal display device, the liquid crystal is injected from an injection port 1013 by a vacuum injection method thereby to fill the display device with itself. After the liquid crystal was fully injected into the liquid crystal display device, the injection port 1013 is closed with an ultraviolet setting type resin 1015 so that the liquid crystal may not leak from the injection port 1013.
As has been described hereinbefore, it can be said that the liquid crystal constructing the liquid crystal display device contacts mainly with the orientation film and the seal member.
Only the orientation regulating force (In the specification, it means a force to regulate the liquid crystal molecules to be oriented in an uniform direction.) of the orientation film has not been sufficient to exert the regulating force so far as to the bulk the liquid crystal so that a homogeneous orientation has been difficult. Therefore, a black display cannot be obtained when no voltage is applied to block the improvement in contrast. Moreover, the smectic liquid crystal such as the ferroelectric liquid crystal or the anti-ferroelectric liquid crystal has a far higher viscosity at the room temperature than that of the nematic liquid crystal. When the orientation of the liquid crystal is partially disturbed, therefore, the orientation of that portion is left disturbed. When the orientation-disturbed region extends to that of the display pixel portion, its contrast is seriously deteriorated.
On the other hand, the liquid crystal in the vicinity of the seal member is subject to the regulating force of the seal member rather than to the regulating force of the orientation film. Even in the display pixel portion, as the case may be, the regulating force of the seal member may be stronger than the orientation regulating force of the orientation film.
Moreover, the smectic liquid crystal, as represented by the ferroelectric liquid crystal or the anti-ferroelectric liquid crystal, must have a very small cell gap (1 to 2 microns). With this cell gap of about 1 to 2 microns, the unnecessary seal member oozes like beard sideway of the pattern which intrinsically should be so. In the serious case, there is found a defect that the unnecessary seal member spreads to the display pixel portion.
Against this problem, there has been adopted the method of mixing a generally spherical material (as will be called the “filler”) having an average particle diameter of about 3 to 4 microns into the resin material to increase the apparent viscosity thereby to prevent the viscosity at the panel heating time from lowering. Where the gap (or the substrate spacing) has to be made smaller, however, there arises a contradiction that the filler obstructs the reduction of the gap. This contradiction could be eliminated if a seal member containing a filler having a small average diameter were used. As a matter of fact, there has not been completed yet the seal member which contains a filler having an average particle diameter of 3 microns or less.